Flexographic printing is a method of direct rotary printing that uses a resilient relief image in a plate of rubber or photopolymer (i.e. a flexographic printing plate) to print articles such as cartons, bags, labels or books. Flexographic printing has found particular application in packaging, where it has displaced rotogravure and offset lithography printing techniques in many cases. While the quality of articles printed using flexographic plates has improved significantly as the technology has matured, physical limitations related to the process of creating a relief image in the flexographic printing plate remain.
In particular, it is very difficult to print small graphic elements such as fine dots, lines, and even text using flexographic printing plates. In the lightest areas of an image (commonly referred to as highlights), the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. Due to the nature of the plate making processes, maintaining small dots on a flexographic printing plate is very difficult. In a pre-imaging (or post-imaging) step the floor of the printing plate is set by area exposure to ultraviolet light from the back of the printing plate. This exposure hardens the photopolymer to a desired relief depth for optimal printing. Floodwise exposure to image-forming radiation via a mask layer followed by a processing step to remove unhardened (i.e. unexposed) photopolymer produces relief dots having a generally conical shape.
The smallest of these dots are prone to be removed during processing, which means no ink is transferred to those areas during printing (the dot is not “held” on plate and/or on press). Alternatively, even if the smallest dots survive processing, they are susceptible to damage on the rotary printer, as small dots often fold over and/or partially break off during printing causing either excess ink or no ink to be transferred.
There remains a need to improve retention of small dots in flexographic printing processes.